As disclosed in Japanese Patent Publication JP2004-307851 A, transparent films have been proposed as a replacement member for a glass plate utilized in a flat-panel display such as a liquid-crystal display, a plasma display, and an organic light emitting diode display.
One example of the transparent film is formed by hot-pressing prepregs each composed of a glass fiber substrate impregnated with a transparent thermoset resin having a refractive index similar to the glass fiber. The combination of the glass fibers and the matrix resin having refractive index compatible to the glass fiber can avoid undue refraction within the transparent film in order to assure good visibility as required for the display.
Generally, an epoxy resin is used as the transparent thermoset resin. In order to approximate the refractive index of the resin to that of the glass fiber, different kinds of epoxy resins one having the refractive index higher than the glass fiber and the other having the refractive index lower than the glass fiber are mixed at a suitable mixing ratio.
When the transparent film is formed by a combination of the epoxy resins having the refractive indexes greater and lower respectively than the glass fiber, minute crack or exfoliation developing at the interface between the epoxy resin and the glass fiber is likely to increase haze factor and therefore lower the visibility. Accordingly, it has been proposed to use an epoxy resin having a low glass transition temperature in order to restrain the minute crack or exfoliation at the interface between the epoxy resin and the glass fiber.
However, since the use of the resin having the low glass transition temperature may reduce heat resistance of the transparent film, it has been a demand of providing the transparent film having a high glass transition temperature in a particular application field of requiring high heat resistance.